The present invention relates generally to signal frequency band control systems and more particularly to a signal freqency band control system providing a high-pass filter or a low-pass filter of variable cut-off frequency. In particular, the system accomplishes an automatic gain control, as the frequency band is varied in accordance with the input signal level.
One prior example of a variable filter circuit capable of varying a signal filter frequency band, uses a variable filter circuit which has, for example, a variable inductance element connected between input and output points. A variable capacitance element and a resistance element are connected in parallel between a junction point of this variable inductance element and the output point and ground (earth). The filter circuit has a slope characteristic of 12 dB/oct.
This variable filter circuit has a characteristic with a variable cut-off frequency in the audio frequency band. For example, if a number of hundred Hz is to be obtained, a variable inductance element having an inductance measured in hundreds H must be used, when an ordinary variable capacitor having a capacitance value measured in hundreds of PF is used, as the variable capacitance element. This circuit has an extremely high cost. Furthermore, when a variable capacitance diode measured in tens of PF is used as the variable capacitance element, a variable inductance element of an extremely large inductance value must be used. In either case, the circuit is not practical.
As another known variable filter circuit, there has been one which has a first low-pass filter comprising a variable resistance element, and a variable capacitance element and a second low-pass filter comprising a variable resistance element and a variable capacitance element, connected in series, and which has a slope characteristic of 12 dB/oct.
This variable filter circuit uses at least two variable resistance elements and two variable capacitance elements. These elements must have a matched characteristic. It is difficult, however, to obtain these kinds of variable elements, with a matched characteristic. On the other hand, if the variable elements deviate in their characteristics, an ideal variable band characteristic cannot be obtained.